Semiconductor-microlaser linewidths

Microlasers composed of a single optical mode interacting strongly with hot, dense electron-hole carriers offer a particularly simple system for analysis. We have studied the lasing-mode linewidth near and well above threshold for semiconductor microdisk lasers. Microdisks with diameters near 2 μm, where the fraction of spontaneous emission coupled into the lasing mode at temperature near 77 K is relatively large, nearly 20%, have been demonstrated. The lasing-mode linewidths measured for these small microdisks are surprisingly large and constant as a function of pump power. Laser linewidths in larger-volume semiconductor lasers vary inversely with pump power, as expected from the Schawlow-Townes limit, and their values can decrease to well below 100 MHz. In contrast, our microdisk line-widths are as large as 80 GHz and remain nearly constant for pump powers well above threshold. These anomalous linewidths, attributed to the nonequilibrium carrier dynamics in the microdisk cavity, provide an interesting test of theoretical models. Our experimental results are compared with a detailed model of the carrier dynamics of the nonequilibrium gain medium in the microdisk cavity. We have developed a simple model to help in understanding the basic mechanisms that determine the lasing linewidths.